1. Field of the invention
The presents invention concerns a process for obtaining precursors for high critical temperature superconductor ceramics for synthesizing superconductor ceramics under optimum conditions of reactivity, yield and homogeneity.
2. Description of the Prior Art
In a prior art method, before synthesis proper, a mixture of precursors is prepared in the form of very fine grains (diameter less than 0.3 mm) by coprecipitation; starting with a solution of the required cations, the precursors are precipitated in the form of salts which are insoluble in water such as oxalates, carbonates, tartrates, citrates, etc. The precipitate obtained in this way is then filtered, washed and calcined at a temperature between 500.degree. C. and 600.degree. C. There results a spongy mass of carbonate or oxide cations which after brief grinding yields a powder consisting of agglomerates of very fine grains with a chemical composition which is highly homogeneous on the microscopic scale; in other words, each agglomerate has the required stoichiometry. From this point of view this process represents a major advance over the grinding of oxides or carbonates prepared separately.
However, a method of this kind applied to the coprecipitation of precursors for superconductor ceramics based on yttrium or bismuth in practise raises problems as described in the following articles:
Mat. Res. Bull vol 23, 1988, 1273 --F. Caillaud PA1 Mat Lett. vol 2, 3, 1990, 105 --C. Y. Shei. PA1 a starting solution of soluble salts is prepared in which the cations are in the stoichiometric proportions of the required superconductor phase, PA1 a first full precipitation is carried out of a first series of cations at a first value of pH, PA1 the first precipitate obtained is filtered out and washed and the filtrate is retained, PA1 the pH of said filtrate is changed to a second value and the residual cations are precipitated, PA1 the second precipitate is filtered, PA1 the first and second precipitates are homogenized, PA1 the product obtained is dried, calcined, and ground.
The working solutions contain a number of unwanted anion and soluble complexes which impede the process of precipitation and can very seriously reduce the yield.
For example, in an acid medium there appear relatively soluble alkaline earth complexes such as EQU Ba(C.sub.2 O.sub.4).sub.2.sup.2-, Sr(C.sub.2 O.sub.4).sub.2.sup.2-, Ca(C.sub.2 O.sub.4).sub.2.sup.2-.
Likewise, in the presence of ammonium a highly soluble copper complex Cu(NH.sub.3).sub.4.sup.2- appears.
Also, if the pH is greater than 2, the bismuth dissolves again in the form of bismuthite ions Bi.sub.2 O.sub.4.sup.2-.
Consequently, if the working solution is neutralized with a non-polluting volatile base such as ammonia or an amine, the copper and the bismuth cannot be fully precipitated because of the alkaline earths and vice versa if the working solution is acidified. Whatever the range of pH of the solution employed, the precipitate departs from the required stoichiometry and the synthesized superconductor phases are impure. It is known that these impurities result in an unacceptable deterioration in superconductive properties including reduced critical temperature and transport current.
To avoid this problem, as is described in detail in the articles mentioned above, users of the process exercise precise control over the pH of the solution during coprecipitation to obtain conditions under which the yield is maximum; also, from the outset they add to the working solution an excess of cations which cannot be precipitated completely. However, it proves very difficult to judge the excess of cations required because the reduction in the precipitation yield depends on a large number of parameters that are difficult to control simultaneously, especially under industrial conditions: pH, temperature, concentration of each species, precipitation kinetics, etc.
An object of the present invention is to provide a process for obtaining all the precursors with the required stoichiometry and reproducibly even under industrial conditions.